Older women who are current users of estrogen replacement therapy and have low bone mass will be enrolled. We postulate that our exercise protocol will cause an adaptive response resulting in a net gain in femoral bone mass in older women through stimulation of bone formation and concomitant but weaker inhibition of bone resorption. We further postulate that we will identify individuals in whom rates of bone turnover are incompatible with this adaptive response. If our hypothesis is proven, exercise may be a reasonable strategy to delay the onset of first fracture in older women who are at risk for hip fracture.